The invention relates to a method and apparatus for the detection of corrosion, pitting, or the like in pipelines whereby, measurements are performed during the travel of a scraper, with the measurement results being stored and evaluated after carrying out the test run, with the apparatus having at least one measuring unit, a processing unit and a recording unit.
In the case of pipelines, an automatic non-destructive testing in connection with corrosion, pitting or the like is necessary. Such faults and defects can be detected by the changes to the thickness of the pipe wall caused by them.
In connection with crack testing and inspection, the eddy current method has been proposed, in which an alternating electromagnetic field of an exciter coil induces eddy currents in the pipeline wall, which are detected by a sensor coil located at the same point. This method has proved satisfactory for the detection of internal cracks, but is unable or is only able with difficulty to discover defects passing from the outer to the inner wall, such as corrosive wall erosion and cracks, due to the generally considerable thicknesses of the pipe walls (approximately 20 mm and more) and the limited penetration depth of the field. A known apparatus operating according to this method is unsuitable for corrosion detection and was also not used for this purpose.
The use of the stray flux method has also been proposed, in which the pipeline wall is magnetized close to magnetic saturation by the use of permanent magnets or electromagnets. The magnetic field locally passes to the outside through cracks in the pipeline wall. The detection of the stray field using suitable magnetic field sensors makes it possible to detect cracks. However, this process is more suitable for detecting cracks than for corrosion measurements and in particular it is not possible to determine the residual wall thickness of pipelines.
Another method is based on the electromagnetic coupling of ultrasonic waves into the pipeline wall and the measurement of the time delay of the ultrasonic pulses reflected on the back wall. Although no ultrasonic coupling medium is required, the method still requires a high power level for coupling in purposes and has a poor efficiency in converting electromagnetic energy into sound energy, so that the signal-to-noise ratio is too low for the requisite fault detection. This method also does not permit wall thickness determination in connection with internal corrosion.